1. Field of the Invention
The invention relates generally to optical amplifiers and in particular to a Tm-doped fiber amplifier pumping scheme.
2. Description of the Related Art
In optical telecommunications networks, high bandwidth is desired for applications such as the Internet, video on demand, and videophone. In many optical communications systems, optical signals having wavelengths in the range 1530-1560 nanometers (nm) are utilized. This wavelength range corresponds to the xe2x80x9cC-bandxe2x80x9d in telecommunications. This wavelength range also corresponds to a minimum attenuation region for silica and silica-based fibers. Optical amplifiers are utilized to amplify the optical signals in those wavelength regions. Conventional optical fiber transmission systems typically exploit erbium doped fiber (EDF) for periodic amplification of signals attenuated by their passage through the optical fiber. In various configurations these erbium fibers can provide amplification in a wavelength range from 1530 to 1620 nm. However, the available low loss region of silica fiber for transmission extends beyond this wavelength range. One of the candidates for extending the amplification bandwidth to the S-Band wavelength region (1450-1530 nm) is the thulium doped fiber amplifier (TDFA). Within the 1450 nm-1530 nm wavelength band, trivalent thulium (Tm3+) has an emission band centered at about 1470 nm. As shown in the Tm3+ energy diagram of FIG. 1, the 3H4-3F4 transition in Tm3+ corresponds to an emission at about 1470 nm.
To date a number of different pumping schemes for Tm3+ doped amplifiers have been demonstrated, including direct 800 nm pumping, 1060 nm up-conversion pumping, dual wavelength 1060 nm+1560 nm pumping and more recently a dual wavelength pumping scheme involving 1405 nm+1560 nm lasers. This last pump scheme has become a preferred pump scheme, primarily because of the intrinsically high power conversion efficiency that can obtained and the possibility of direct diode pumping of the fiber amplifier.
However it is not clear that a suitable commercial amplifier can be made from this pump scheme, in particular because of the intrinsically high noise figure that accompanies in-band (or resonant) pump schemes, where the pump and signal wavelengths are very close together.
The present invention is directed to overcoming or at least reducing the aforementioned noise figure problems, namely achieving a low noise figure without sacrificing the high power conversion efficiency that can be obtained with this pump scheme.
In view of the foregoing, according to one embodiment of the present invention, an optical amplifier comprises a trivalent thulium-doped optical fiber; a first pump light emitting device optically coupled to the fiber and acting as the primary pump source at a first wavelength, and a second pump light emitting device optically coupled to the fiber and acting as a secondary pump source at a second wavelength. In a preferred aspect of the present invention, the amplifier also includes a third (also referred to herein as an auxiliary) pump light emitting device optically coupled to the fiber and acting as a third pump source, specifically at a third wavelength. Each of the first and second pump sources comprise at least one pre-selected wavelength to pump an excited state of the fiber. The first pump source has a wavelength pre-selected to provide a reduced noise figure for the amplifier. The second pump source has a wavelength pre-selected to increase the optical efficiency of the amplifier. The third (auxiliary) pump source can have a wavelength pre-selected to populate the 3F4 energy level of the fiber, without substantially increasing amplified spontaneous emission of the 3H4-3H6 transition.
According to another embodiment of the present invention, a method of amplifying an optical data signal comprises providing a thulium-doped optical fiber for transmitting an optical data signal, generating a first amplification pump wavelength corresponding to a first emission cross section of the 3H4 to 3F4 transition of the fiber, and generating a second amplification pump wavelength corresponding to a second emission cross section of the 3H4 to 3F4 transition; introducing said first amplification pump wavelength and said second amplification pump wavelength into said fiber, wherein the first wavelength corresponds to a lower emission cross section and lower excited state absorption cross section of the fiber than the second wavelength.
Other advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.